1. Field of the Invention
The present invention generally relates to sheetmaking systems and, more particularly, to sheetmaking control systems wherein measuring devices scan across travelling sheets during manufacture.
2. State of the Art
It is well known to make on-line measurements of properties of sheet materials during manufacture. The purpose of on-line measurements, generally speaking, is to enable prompt control of sheetmaking processes and, thus, to enhance sheet quality while reducing the quantity of substandard sheet material which is produced before undesirable process conditions are corrected. In practice, most sheetmaking machines have been instrumented to include-on-line sensors. In the paper-making art, for instance, on-line sensors detect variables such as basis weight, moisture content, and caliper of sheets during manufacture.
On-line measurements during sheetmaking are, however, difficult to make accurately. One factor affecting on-line measurement is that many sheetmaking machines are large and operate at high speeds. For example, some paper-making machines produce sheets up to four hundred inches wide at rates of up to one hundred feet per second. Another factor affecting on-line measurements is that physical properties of sheet materials usually vary across the width of a sheet and may be different in the machine direction than in the cross direction. (In the sheetmaking art, the term "machine direction" refers to the direction of travel of a sheet during manufacture, and the term "cross direction" refers to the direction across the surface of a sheet perpendicular to the machine direction.)
To detect cross-directional variations in sheets, it is well known to use on-line scanning sensors that periodically traverse back and forth across a sheetmaking machine in the cross direction. Normally, measurement information provided by each scanning sensor is assembled to provide, for each scan, a "profile" of the detected property of the sheet. In other words, each profile is comprised of a succession of sheet measurements at adjacent locations extending generally in the cross direction. Based upon the profile measurements, variations are detected in sheet properties in the cross-direction and appropriate controls are adjusted with the goal of providing uniform cross-directional profiles, i.e., profiles that have constant amplitude in the cross direction.
In actual practice, although scanning sensors travel rapidly across sheetmaking machines in the cross direction, consecutive measurement points are not aligned exactly in the true cross direction; that is, the actual points at which scanning sensors provide measurements are not aligned exactly perpendicular to the edge of the sheet being measured. Instead, because of sheet velocity, scanning sensors actually move diagonally across the surface of a travelling sheet with the result that consecutive scanning paths follow a zig-zag pattern. Therefore, profiles based on sheet measurements taken by scanning sensors along the zig-zag paths include some machine-direction variations.
As a result, when consecutive cross-directional profiles are compared or when one location on a profile is compared to another location, machine-direction variations can be confused with cross-directional variation. In the sheetmaking art, such confusion of machine-direction and cross-direction measurements is referred to as MD/CD coupling. As a result of MD/CD coupling, control systems that are intended to control cross-directional variations sometimes introduce artificial control disturbances which worsen, rather than improve, sheet uniformity in the cross direction.
Currently, sheetmaking control systems either do not compensate for MD/CD coupling or employ filters that average errors. Such filtering is not totally satisfactory for several reasons, including the fact that the filtering necessarily entails the loss of otherwise useful measurement information.